In the field of nuclear medicine, images of the internal structures or functions of a patient's body are formed by using gamma cameras to detect radiation emitted from within the body after the patient has been injected with a radiopharmaceutical substance. A computer system generally controls the gamma cameras to acquire data and then processes the acquired data to generate the images. Nuclear medicine imaging techniques include Single-Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET). SPECT imaging is based on the detection of individual gamma rays emitted from the body, while PET imaging is based on the detection of gamma ray pairs that are emitted in coincidence in opposite directions due to electron-positron annihilations. PET imaging is therefore often referred to as "coincidence" imaging.
One factor which has a significant impact on image quality in nuclear medicine is non-uniform attenuation. Non-uniform attenuation refers to the attenuation of radiation emitted from an organ of interest before the radiation can be detected. Such attenuation tends to degrade image quality. A technique which has been used to correct for non-uniform attenuation is transmission scanning, in which gamma radiation from a transmission source is transmitted through the patient to a corresponding scintillation detector and used to form a transmission image. The transmission images provide an indication of the amount attenuation caused by various structures of the body and can therefore be used to correct for attenuation in the emission images.
Many gamma camera systems include multiple detectors, multiple transmission sources, or both. In such systems, a particular transmission source may be directed at a particular "target" detector at any given point in time. One problem which may be encountered with such systems is that of adequately shielding a transmission source from the detectors which it is not supposed to be illuminating. For example, the correct interpretation of detected transmission radiation may depend upon knowing the location of the source of that radiation. Radiation detected by a detector that is not the intended target is referred to as transmission self-contamination. Transmission self-contamination introduces inaccuracies into the transmission scan and therefore degrades the quality of the emission images that are ultimately corrected by the transmission data. Even with proper shielding, it may be impossible to completely prevent transmission self-contamination. Hence, it is desirable to have a technique for characterizing and correcting for transmission self-contamination in a nuclear medicine imaging system.